Gas turbine engines have a number of complex components that require periodic inspection. It has been found quite useful to conduct these inspections, when possible, without tearing apart the engine. For example those gas turbine engines installed on aircraft are periodically inspected using a technique known as a borescope inspection. Borescope inspections typically involve the insertion of a viewing apparatus, a borescope, from the engine exterior, through an access port to some interior portion of the engine. In a typical arrangement a borescope includes a flexible wand that carries a light source and a vision means, such as a video camera. The wand is usually flexible and can be manipulated such that the borescope tip can be directed toward a target, in a desired direction. The light and vision means then allow an operator, positioned at some point remote from the engine, to view the desired point of the engine interior on a video screen. Turbine blades are one such engine component that are inspected periodically through borescopes for signs of cracking or deterioration. However, this general method of inspecting gas turbine engines has not been expanded to other kinds of engine repair and service. There is a need to adapt such methods of engine access to other repairs.
The modern jet aircraft is a very high capital thing. Demurrage costs and lost revenue potential that arise when an aircraft is out of service add to its operating cost. Thus maintenance and repair strategies associated with aircraft, and turbine engines in general, seek methods that have a quick turn around. The goal is to return the vehicle to service as quickly as possible consistent with quality and safety demands. There is a continuing need for improved repair methods that allow quicker and faster repairs that minimize the time that any vehicle is out of service.
Gas turbine engines include many components with complex shapes. Impellers and blisks for example have airfoils with surface curvature that extends in three dimensions. Impellers and blisks are being increasingly specified in modern design as a method to achieve high compression in relatively short lateral spaces. Additionally gas turbine enegine components, and especially impellers and blisks, are often fabricated from expensive alloy materials. Typical alloys include the class of material known as superalloys, alloys having a high level of nickel and/or cobalt. The complex design, and expensive materials, that are used to fabricate impellers and blisks often means that they can be quite expensive.
As a consequence of these design and material criteria, it is desirable to repair damaged components when possible. Various welding operations, including laser welding, have been developed to treat damaged components. Welding operations may include repair of cracked materials as well as material deposition to restore worn parts.
Welding operations used with impellers, blisks, and other complex geometries often benefit from computerized control of the welding operation. Computer programs that contain the geometry of the device are able to perform a welding operation in an automated fashion, often in a manner superior to that of human controlled weldng. The uniformity and reproducibility in computer-controlled welding operations is additionally desired as a way to eliminate the variability that comes in iterative human processes. This automated welding is most efficient when human interaction with the welding operation is held at a minimum.
However, the removal of the human component from a welding operation can itself lead to difficulties. One problem encountered with automated welding of turbine blisks and impellers is damage to areas proximate to the target zone of the work piece. Whereas a human operator could observe when damage is occurring and take steps to minimize the damage, a machine may not be able to do this. Thus, it would be desired to develop methods for computer-assisted welding that minimize welding-related damage.
Welding, as often practiced in automated welding repair of gas turbine engine components, often follows certain procedures and uses materials that can contribute to work piece damage. For example, the temperatures that are required to successfully weld superalloy materials are relatively high welding temperatures. The heat that is transferred into the substrate material and to the filler material (powder or wire feed) can affect neighboring areas of the work piece. Splatter damage can also result from heated materials at the target area impinging on other areas of the work piece. Again, it would be desired to develop techniques and materials that minimize this damage.
Additionally, an inert gas such as argon is often used in welding operations. An inert gas, which does not react with welding materials and base materials, minimizes the presence of oxygen and thus minimizes oxidation reactions, which can potentially weaken the weld. The inert gas can itself be heated to very high temperatures during the operation. The heated gas can warp or otherwise damage non-target areas of the work piece with which it comes into contact. This particularly occurs when a relatively large volume of heated gas reaches neighboring area. Consequently it would be desired to develop materials and techniques that minimize weld related damage arising from large volumes of heated inert gas.
The geometry of turbine engine blisks and impellers makes them particularly vulnerable to welding-related damage. (Blisk is the term used in the aeronatutical field for a unitary piece with a rotor and airfoils.) A blisk, for example, contains a series of airfoils that radiate out from a central hub. The airfoils may be in close proximity to one another, across the blisk. This close positioning of airfoils subjects neighboring airfoils to damage when heated debris or gases impinge on them.
Accordingly there is a need for an apparatus and method to protect airfoils from welding damage. It is desired that the apparatus be able to shield or protect airfoils and blisks from harm when neighboring airfoils are being welded. Further, it is desired that the apparatus, and method of using the apparatus, be suitable for use with automated welding systems. It is thus desired that the efficiency of automated welding systems not be unduly compromised by the protective apparatus and method. The present invention addresses one or more of these needs.